Influenza and other emerging viruses represent major health concerns worldwide, and current strategies for prevention or treatment of disease are insufficient. Type I interferon is a cytokine that induces an antiviral state in cells by transcriptionally upregulating hundreds of genes. Some of these genes encode proteins with direct antiviral activity, though only a small number of these proteins have been mechanistically characterized. Among these proteins, the interferon-inducible transmembrane protein 3 (IFITM3) has been recently shown by us and others to have broad antiviral activity against all subtypes of influenza virus tested as well as a number of other virus families. Furthermore, this protein is unique among interferon effectors in that it appears to act by preventing entry or fusion of viruses rather than inhibiting viral replication. Using chemical reporters, we have shown that IFITM3 is post-translationally palmitoylated and this lipid modification regulates its antiviral activity. Furthermore, our preliminary data indicate that IFITM3 is also ubiquitinated. The overall goal of the proposed research is to increase understanding of innate antiviral immunity by characterizing the mechanism of action of IFITM3 and its cellular regulation by post-translational modifications. In the K99 phase, we will develop biochemical and microscopy assays to test the hypothesis that palmitoylation controls proper trafficking of IFITM3 allowing it to interact with viral particles and prevent viral entry. With this knowledge in hand we will then seek in the R00 phase to identify enzymes responsible for IFITM3 palmitoylation and look at the global transcriptional regulation of palmitoylating enzymes during viral infections. Furthermore, we will apply assays developed in the K99 phase to understand the role of ubiquitination in IFITM3 biology and its interplay with palmitoylation. Analyzing the mechanism of action of IFITM3 and the control of this activity by a unique set of post-translational modifications may provide insights necessary for harnessing the power of type I interferon for combating viral pathogens.